The present invention relates generally to stepped square quadrature amplitude modulation (SS-QAM) system demodulators, and more specifically to an analog-to-digital converter for use with SS 2.sup.2n -QAM demodulators.
U.S. Pat. No. 4,675,619 describes a stepped square 2.sup.2n -QAM demodulator which comprises an (n+2)-bit analog-to-digital converter for each of the I- and Q-channel systems to convert the (2.sup.n +2) levels of the demodulated analog signal to a corresponding digital signal containing error indicating least significant bits. For example, the SS-256-QAM system has a signal constellation of stepped square configuration with 18 rows extending along the Q-channel axis and 18 columns extending along the I-channel axis. Each of the rows must be discriminated by comparison with 35 dicision thresholds of the Q-channel system and each of the columns must also be discriminated by comparison with 35 decision thresholds of the I-channel system. Because of the need to generate error signals for feedback control, the analog-to-digital converter of 6 bits or more would be required. To implement such an analog-to-digital converter, 2.sup.6 comparators are required, i.e., the (n+2)-bit analog-to-digital converter requires as many as 2.sup.n+2 comparators. One approach to this problem would be to compress the amplitude of the demodulated signal. However, discrimination error will increase due to a possible reduction of signal to noise ratio. Another approach is to employ a 6-bit analog-to-digital converter having 63 or more decision threshold levels and a high speed read-only memory connected to the output of the 6-bit A/D converter. The read-only memory has data stored in locations which are addressable as a function of the input from the A/D converter, the retrieved data indicating signal points located inside or outside of a square of the signal constellation of a SS-256 QAM system. However, a large chip size will result if the A/D converter is implemented by LSI technology and a substantial amount of power is dissipated.